There are many instances in the use of astronomical telescopes in which it is necessary or desirable to rotate the telescope on its polar axis at a constant rate. For example, because of the rotation of the earth, it is necessary to rotate a telescope on its polar axis at a substantially constant rate of one revolution per day when making long time studies or photographs of celestial bodies.
A variety of telescope-mounting systems have been devised. One of the most common telescope mounting systems is known as the equatorial mount. The equatorial mount embodies a two axis system having an inclined rotational axis which parallels the earth's rotational axis, and having a second rotational axis which intersects the first axis at right-angles. The inclined axis is referred to as the polar axis and the other axis is referred to as the declination axis. Rotation of the mounting unit about the declination axis adjusts the declination angle of the telescope. Rotation of the mounting unit about the polar axis moves the telescope in a rotary tracking motion. The equatorial mounting system is advantageous for the reason that it permits tracking of celestial objects by simple rotation about the polar axis which is fixed at a given angle corresponding to the latitude at which the telescope is located.
Accordingly, the most common astronomical telescope mounting unit is the equatorial platform one in which the telescope is mounted for rotation about the polar axis which is parallel to the celestial polar axis, and for rotation about the declination axis which is perpendicular to the polar axis. Such mounting units commonly include a motor for rotating the telescope about its polar axis at a rate of one revolution per day to track the observed celestial object as it rotates about the celestial pole, the latter rotation being the visible effect of the earth's rotation about its axis.
For smaller and medium size telescopes, as mentioned above, the most common form of mounting unit is the German equatorial platform on which the bearings for the declination assembly are attached outboard of the upper bearing of the polar axis assembly and the telescope is, in turn, attached outboard of the declination axis bearing. A counterweight is mounted at the opposite end of the declination shaft so as to maintain balance in all positions of rotation about the polar axis. While this type of mounting unit has the advantage of being fairly light, it suffers from the disadvantage that large mechanical stresses are placed on the shafts. It is therefore necessary to make the shafts sufficiently strong to resist vibration and to place the two bearings fairly well apart to insure stability.
A second equatorial mounting assembly which eliminates the need for the counterweight is the fork type. In the fork mounting, the polar axis is similar to that in the German equatorial mount, but the telescope is carried between the declination axis bearings in a fork through which it is swung to gain complete accessibility to the sky. Although the counterweight and heavy declination shaft have been eliminated in the latter assembly, the fork itself must be of a massive structure thereby requiring a substantial polar axis assembly to support the load.
Examples of astronomical telescope mounting units and drive systems of the type described above may be found, for example, in U.S. Pat. Nos. 2,326,552; 3,136,388; 3,603,664; 3,606,520; 3,751,134; 3,840,284; 3,885,858 and 3,893,746.
The objectives of the present invention are to provide an improved and inexpensive astronomical telescope mounting unit and drive system of the German equatorial or fork type, in which an astronomical telescope is rotated about the polar axis at a desired rate by a motor which is coupled through a worm to a worm gear sector, rather than to an entire worm gear. This technique permits high precision accuracy to be achieved between the worm and the worm gear sector by utilizing relatively large effective worm gear diameter, and since only a worm gear sector, rather than an entire worm gear is used, space is conserved.
Unlike the prior art drives, the rotation of the telescope about the polar axis in the unit of the invention is not continuous, but is limited to a particular time interval during which the worm moves the worm gear sector from one of its ends to the other. If the observation process cannot be completed within that particular time interval, the segment must be reset. The time interval can extend, for example, up to two hours, which is sufficient for most observations and astronomical photographs. However, if more time is required, such as for photographing a faint distant star, the worm gear can be reset on the worm in a matter of seconds. The worm is preferably made of special proprietary bronze alloy so as to have molecularly stable high precision threads.
A microswitch is provided, as will be described, to permit the observation process to be terminated automatically at any set time within the aforesaid time interval, so that there is no need to monitor the time continually. A five minute prealarm, for example, may also be provided by the same or a different microswitch to tell the operator that the instrument is nearing the end of the preset time interval.
The mount of the invention is manually adjustable about a horizontal axis perpendicular to the polar axis so that it may be set to correspond to the latitude at which the instrument is used. A plumb bob scale is provided which may be calibrated directly in latitude, so as to facilitate the adjustment of the instrument.
As will be described, the assembly of the invention has a modular construction, so that it may be used as a German equatorial or fork mount to accommodate a variety of sizes and types of telescopes, merely by removing, for example, four screws.